HOW: Lie on your back and put both of your feet on the swissball. With your knees straight, perform a bridge by digging your heels into the swissball to lift your hips up. You can use your arms are your side to stabilize your self if you need it. The goal is to move straight up and down without any side to side motion.
FEEL: You should feel your hamstrings working very hard to lift your hips up and stabilize the swissball. You will also feel your core engaged to keep your body from moving.
COMPENSATION: Do not let the swissball move. Make sure to only extend at the hips, and not the lower back!